1. Description of the Prior Art
What is called a YIG (yttrium, iron, garnet) bandpass filter has been used in microwave circuits. It may be comprised of input and output conductors (i.e. transmission lines) in different parallel planes that intersect at 90.degree. when viewed in a direction perpendicular to the planes. A sphere or disc of YIG material is located between the conductors at the point where they intersect. Ordinarily, there would be no coupling between the orthogonal conductors, but the presence of an applied DC magnetic field passing through the YIG sphere or disc in a direction perpendicular to the planes permits coupling to occur in a band of frequencies determined by the strength of the applied magnetic field.
At present, such filters are constructed as discrete units that are normally expensive and rather bulky. When they are used with microwave integrated circuits (MIC's) that are formed on a substrate such as GaAs or Si, they must be mounted on the substrate and electrically connected to the circuits therein. Thus, they are not integral and are basically incompatible with the MIC.